Tycho Brahe

Tycho Ottesen Brahe was born into a highly aristocratic, very wealthy family on December 14, 1546. Tycho was the second of the couple’s 12 children. Something rather remarkable happened to Tycho in his second year of life – he was kidnapped by his uncle and aunt, Jørgen Brahe and Inger Oxe, when his parents were away from home. Tycho’s uncle and aunt were childless, and they believed that Jørgen was entitled to a lawful son and heir to his estates. Tycho’s natural parents eventually agreed to this, so Tycho was raised by his uncle and aunt as if he were their own son.

  Tycho’s foster mother, Inger, had come from an academic family and she persuaded her husband that Tycho should receive an academic education. Tycho began school aged six or seven, a grammar school where he probably learned the classical languages, mathematics, and the Lutheran religion.

In April 1559, aged 12, Tycho matriculated at the University of Copenhagen. He studied a general classical curriculum for three years, during which time he became increasingly absorbed in astronomy. He bought a number of important books in the field, including Johannes de Sacrobosco’s On the Spheres, Peter Abian’s Cosmography, and Regiomantus’s Trigonometry.

Tycho’s interest in astronomy began with the solar eclipse of August 21, 1560. In Copenhagen this eclipse was barely noticeable – less than half of the sun was covered. The eclipse inspired Tycho not because it was spectacular, but because astronomers had predicted exactly when it would happen. Tycho was fascinated, and wanted to learn how he too could make predictions like this.

Using just a basic, fist-sized celestial sphere and string, Tycho discovered that tables of predictions of planet positions sourced from the works of both Ptolemy and Nicolaus Copernicus were rather unsatisfactory.

In August 1563, aged 16, Tycho began his first logbook of astronomical observations. He observed a one-in-twenty-year conjunction of Jupiter and Saturn, and again noted errors in both Copernicus’s and Ptolemy’s predictions. Using Ptolemy’s data tables, the conjunction timing was wrong by a month!

It became Tycho’s goal to produce truly accurate predictions of planetary positions based on accurate observations. Although he did not quite succeed in his ambition to make all his measurements accurate to within one arc minute, many of them did meet this standard, and his observations were a phenomenal five times more accurate than his peers made.

Tycho made his first significant discovery on November 11, 1572. Observing the night sky from an uncle’s home, Tycho was amazed to see a new light brighter than Venus in the sky.

He studied the new heavenly body for a year. He deduced that it was a star because, unlike closer bodies such as the planets, its position relative to the other stars did not change.

In 1573, Tycho’s name became well-known in astronomical circles when he published De nova stella – The New Star. Although other people had also observed the new star, Tycho published the most comprehensive study of it. Tycho’s new star gradually faded until, after a year, it was no longer visible to the naked eye.

The Great Comet of 1577 made people fearful, because comets were seen as bad omens.

Tycho recorded the comet’s positions between November 13, 1577 and January 26, 1578, after which he could no longer see it.

Tycho used Hipparchus’s parallax method to measure the comet’s distance from the earth.

Unfortunately, there was insufficient parallax for him to pin down the distance, but he was able to say that:-

The comet was much farther away from our planet than the moon is – at least six times as far. This refuted the popular idea that comets traveled within the earth’s atmosphere.

The comet’s tail always pointed away from the sun.

The comet’s path was associated with the sun, not the earth.

The comet had another far-reaching consequence for Tycho and science. It prompted him to begin making observations with a view to producing his own star catalogue to replace Ptolemy’s ancient work.

Tycho accurately recorded the positions of 777 stars by 1592, and he eventually amassed data for 1,006 stars. Tycho’s catalog was later worked on and published by Johannes Kepler. 

In Prague, Tycho gave Johannes Kepler a job as his assistant. Together, they began working on a new star catalog, but it was slow work. Tycho Brahe died aged 54 on October 24, 1601 in Prague. The catalog was eventually published by Kepler in 1627 as the Rudolphine Tables. These were by far the most accurate astronomical data tables ever published, with planetary data and 1,006 star positions. The majority of stars were cataloged to within one arc minute accuracy, which had been Tycho’s ambition.

Kepler's Laws of Planetary Motion

Kepler was assigned the task by Tycho Brahe to analyze the observations that Tycho had made of Mars. Of all the planets, the predicted position of Mars had the largest errors and therefore posed the greatest problem. Tycho's data were the best available before the invention of the telescope and the accuracy was good enough for Kepler to show that Mars' orbit would precisely fit an ellipse. In 1605 he announced The First Law:

Planets move in ellipses with the Sun at one focus.

The radius vector describes equal areas in equal times. (The Second Law)

Kepler published these two laws in 1609 in his book Astronomia Nova.

For a circle the motion is uniform, but in order for an object along an elliptical orbit to sweep out the area at a uniform rate, the object moves quickly when the radius vector is short and the object moves slowly when the radius vector is long.

On May 15, 1618 he discovered The Third Law:

The squares of the periodic times are to each other as the cubes of the mean distances.

This law he published in 1619 in his Harmonices Mundi . It was this law, not an apple, that led Newton to his law of gravitation. Kepler can truly be called the founder of celestial mechanics.